Ink jet printer ink for printing on ceramics or glass

ABSTRACT

A pigmented ink usable in an ink jet printer in which the maximum particle size of the pigment is sufficiently small not to block the nozzles or the filters of the printer and the particle size range is sufficiently narrow for the ink to have a low viscosity for the printer to operate. The ink is particularly suited for printing on ceramics or glass.

This application is a PCT application. This application claims thepriority date of Feb. 27, 1991 for Great Britain Patent No. GB9104171.5.

This invention concerns improvements in or relating to inks andparticularly but not exclusively pigmented inks for use in ink jetprinting.

Modern trends in printing are towards non-contact systems using computergenerated and/or computer controlled designs. Such a system is Ink JetPrinting which is used widely through out the textile, paper and foodindustries. Ink Jet Printing offers major advantages as outlined below:

1. It is non-contact which means the printer does not at any time comein contact with the substrate and since only the drops of ink areprojected onto the surface then the print design can be applied to anyplane surface and to uneven and delicate surfaces. The technique isalready used to print onto delicate foodstuffs e.g. eggs, cakes etc.Also printing distances can be anything from 1-20 mm depending on thequality required. The usual is from 1-5 mm so surfaces such ascorrugated cardboard present no problems.

2. Versatility--Designs can be computer generated and/or computercontrolled. This means that designs can be changed rapidly and can bemanipulated, magnified or reduced as desired. With such a system it ispossible carry out short runs or to change from design to designrapidly, as no down time is required

3. High speed--The actual speed achieved depends on various factorsincluding the ink properties and the quality required but typical speedsare between 0.25 and 6 meters/see.

4. Multicolour--Theoretically there is no limit to the numbers ofcolours that can be used providing one has a computer powerful enough tocontrol the jets. Recent developments have produced 100 nozzle systemsgenerating over 6 million drops/sec. all being controlled by a computer.

5. Reliability--Ink jet printers have no moving parts and hence they areinherently reliable. At present there are systems which are workingcontinuously for 100 h/week without any down time.

6. No ink wastage--Any ink not used is recirculated to be used again.The only ink used is on the design.

Up to now, the inks used in these ink jet printers have been based onsoluble organic dyes usually in an organic medium. The reason for thisis that the ink jet printing nozzle is very small usually between 10 and100 microns and it has not been technically feasible to consider the useof highly pigmented inks in these systems, as the solid particlesrapidly block the system and prevent flow.

The soluble dye inks work well in ink jet printing but suffer from themajor drawbacks of non-permanency (fading in daylight) afterapplication. They tend to fade rapidly and after a time virtuallydisappear.

There is a need, therefore, for permanent inks and one way to achievethis is to use pigmented inks, where the pigment is usually a lightstable, inorganic or organic colour.

In certain applications the need to use pigmented inks is paramount. Forexample in the decoration of ceramics and glass, pigmented inks areessential. In most other printing applications once the design has beenprinted, the decoration process is complete. However, in ceramics andglass decoration, printing is followed by a high temperature firingafter which only the pigment and flux remains to form the permanentdecoration, all media and organics are burnt off.

In certain other applications e.g. computing and banking a `magnetic`ink is required usually comprising of a ferromagnetic pigment in amedium which is capable of being read rapidly by a magnetic sensor.

Other applications require resistance to fading (Permanency). Forexample legal documents, household textiles and artifacts e.g. wallpaperetc. In all these applications a high degree of permanency is requiredand this can only be achieved by the use of pigmented inks.

Still further, certain colours e.g. opaque can only be achieved by theuse of pigments.

Typical ranges of physical parameters required of an ink for it to besuitable for ink jet printing are shown below.

    ______________________________________                                        Viscosity         1-100 mPas                                                  Yield Value       0-10 Pa                                                     Conductivity      150-16000 μmhos/cm                                       Surface Tension   20-70 dynes/cm                                              pH                4-11                                                        ______________________________________                                    

As can be seen, the viscosity generally has to be low, to ensure thatthe ink will flow easily through the nozzle without having to applyexcessive pressure which can cause splash back or `splatter` from theworkpiece. There can also be trouble with break up into dropletmodulation. The surface tension also has to be within limits to allowuniform break up into equally spaced equally sized droplets. Thisparameter, however is not so critical as the conductivity which ideallyis within the range 150-16000 μmhos/cm to enable the droplets to becharged satisfactorily. The pH should be maintained between 4 and 11 toavoid damage to the equipment.

The presence of large amounts of pigment particles in the ink canseriously affect any of these parameters. The introduction of pigmentadditions to these systmes normally increases viscosity to unacceptablelevels. Also if the pigment is poorly dispersed this can lead tovariable modulation and charging, often the pigment itself is moreconducting than the medium and poor disperson can lead to variableconductivity which in turn gives erratic charging of droplets and lossof printing control. Furthermore many inorganic pigments tend to be veryhard materials and at such high loadings abrasion of the nozzles can bea problem. All these factors can lead to variable ink jet printingcharacteristics and all have to be controlled precisely to ensure goodquality printing.

In the specification all the percentages shall be expressed as weightpercent.

According to the present invention there is provided an ink comprisingan inorganic pigment and usable in an ink jet printer for printing on toceramics or glass to provide a decoration thereon following subsequentfiring in which the maximum particle size of the pigment is sufficientlysmall not to block the nozzles or the filters of the printer, and theparticle size range is sufficiently narrow for the ink to have a lowenough viscosity for the printer to operate.

The median particle size of the pigment is preferably between 0.2 and 2microns and desirably the pigment has a surface area of between 4 and 30m² /g.

For use with a 50 micron nozzle the maximum particle size of the pigmentis preferably below 4 microns, and desirably less than 20% of theparticles of pigment are less than 0.2 microns.

Pigments other than carbon black or titania based pigments arepreferably provided in the ink. The proportion of pigment in the ink ispreferably in the range 1-85% and desirably 20-60%.

The ink preferably comprises greater than 15% of a medium. The medium ispreferably a polar solvent and can be any of water, methyl ethyl ketone,alcohol or a mixture of these,

The ink also preferably comprises one or more of the followingconstituents in the range given:

    ______________________________________                                                           WT. %                                                      ______________________________________                                        Surfactant/Dispersing agent                                                                        0-20                                                     Suspending Agent     0-10                                                     Film Former/Binder/Hardener                                                                        0-20                                                     Humectant            0-20                                                     Chelating agent      0-1                                                      Biocide              0-1                                                      Defoaming agent      0-1                                                      Buffering agent      0-1                                                      Anti wicking agent   0-20                                                     or                                                                            Substrate penetrant  0-5                                                      ______________________________________                                    

The surfactants/dispersing agents may be wetting agents and maycomprise:

For aqueous systems, sodium polyphosphate and/or sodium carboxylic acid;

For organic systems, triethanolamine and/or dimethylethanolamine salt.

The film formers/binder/hardener may comprise one or more of:

resins, rosins, esterified resins, acrylic, styrene acrylic, cellulose,amides, epoxies.

The humectant may be polyethylene glycol (PEG). The chelating agent maycomprise EDTA.

The biocides may be Formalin or one of the acticide range of biocides.The defoaming agents may comprise low molecular weight silicones.

The invention also provides an ink usable in an ink jet printer forprinting on to ceramics or glass to provide a decoration thereonfollowing subsequent firing, the ink comprising an inorganic pigmentaccording to any of the preceding ten paragraphs with a particle sizesufficient to maintain a required colour intensity; and a Flux also witha maximum particle size sufficiently small not to block the nozzles orfilters of the printer and a particle size range which is sufficientlynarrow such that the ink has a low enough viscosity to operate in theprinter.

The invention will now be described in more detail and with reference toexamples of inks according to the present invention.

The properties of the pigment used, are crucial to performance. The mainparameters for characterising the pigment (or flux) are:

1. Median particle size

2. Particle size distribution

3. Surface area

The median particle size should be between 0.2 and 2 microns and surfaceareas between 4 m² /g and 30 m² /g are desirable.

The maximum particle size is dependent on the filters and nozzle usedbut generally should be less than 4 microns for a 50 micron nozzle.Greater than this the particles tend to clog the filters and block thenozzle. Also the larger particles have more tendency to sediment in thelines or print head causing erratic behaviour and eventual blockage.

The minimum particle size is determined by the viscosity limit in thespecification. The finer the particles the higher the viscosity becomesand thus too many fine particles can be undesirable. Typically for a 50micron nozzle less than 20% of the particles should not be less than 0.2microns.

Therefore what is required is a pigment with a narrow particle sizedistribution as is shown in the accompanying figure. This distributioncan only be achieved using special grinding techniques, classificationor precipitation.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying FIGURE shows a particle size distribution for aninorganic pigment usable in an ink according to the invention. Thevertical axis shows cumulative mass percent whilst the horizontal axisshows equivalent spherical diameter in microns.

The actual size range used in practice is determined by the nozzlediameter and filters used and these are selected according to thequality of printing required.

As well as closely monitoring the particle size distribution of thepigment the characteristics such as viscosity or yield value of the inkcan be improved by the provision of one or more additional components asoutlined below.

1. Surfactant/Dispersing agents--to facilitate even distribution ofpigment throughout the medium to ensure uniform modulation and charging.

2. Suspending agents--to stabilize the ink by preventing sedimentationof the pigment suspension.

3. Film formers, binders and hardeners to keep colour together when dryi.e. to prevent powdering, and to give handleability and scratchresistance of the print.

4. Humectants to prevent premature drying of the ink and build up ofsolids around nozzle.

5. Chelating agents--these are present to control the effect ofpolyvalent ions.

6. Biocides--to prevent bacterial action and subsequent degradation ofthe system.

7. Defoaming agents--to prevent bubble entrapment which leads to jetinstability.

8. Anti-wicking agents--to control spread of ink into the substrate.

9. Substrate penetrants--enhance penetration of ink into substrate.

10. Buffering agent--to maintain pH within limits.

The type and quality of each of these components must be determinedwhilst still maintaining the overall characteristics of the ink requiredfor ink jet printing. Some of these additives fulfil more than onefunction. A general requirement is that any organic compounds usedshould have fairly short polymer chains so as to allow complete anduniform break up into spherical droplets, Organic compounds with longpolymer chains tend to cause `stringing` i.e. the droplets form longstrings of beads which prohibits satisfactory charging and printing.

Listed below are the composition (in weight parts) of three inksaccording to the present invention.

    ______________________________________                                                              % Weight                                                ______________________________________                                        Example 1.                                                                    An ink containing 20% by volume of an inorganic pigment                       Inorganic green pigment -                                                                             44.08                                                 mean particle size 0.5 microns                                                Deionized Water         42.38                                                 Dispex N40 (surfactant/dispersant)                                                                    0.42                                                  Bentonite (suspending Agent)                                                                          0.04                                                  Vinyacryl 7170          3.39                                                  50% by wt. styrene acrylic                                                    copolymer suspension                                                          (binder/film former)                                                          Polyethlene glycol 200 (PEG) (humectant and                                                           8.47                                                  anti-wicking agent)                                                           E.D.T.A (chelating agent)                                                                             0.21                                                  Acticide BG (biocide)   0.08                                                  50 Cst silicone oil (defoamer)                                                                        0.04                                                  Alkyl glycol ether (substrate penetrant)                                                              0.85                                                  Sodium carbonate (buffer)                                                                             0.04                                                  Example 2                                                                     An ink containing 10% by Volume of an inorganic pigment                       Inorganic green pigment  32.55                                                Deionized Water          57.87                                                Dispex N40 (surfactant/dispersant)                                                                     0.36                                                 Polyethylene glycol 200 (PEG) (humectant and                                                           9.04                                                 anti-wicking agent)                                                           E.D.T.A. (chelating agent)                                                                             0.18                                                 Example 3                                                                     An ink containing 20% by volume of an inorganic yellow                        pigment                                                                       Inorganic yellow pigment 47.02                                                Deionized water          33.36                                                Isopropyl alcohol        8.34                                                 Solsperce 20,000 (surfactant/dispersant)                                                               0.44                                                 Low molecular polyvinyl alcohol                                                                        0.89                                                 (binder/film former)                                                          Polyethylene glycol 200 (humectant)                                                                    4.44                                                 E.D.T.A (chelating agent)                                                                              0.53                                                 Acticide BG (biocide)    0.09                                                 50 cst silicone oil (defoamer)                                                                         0.01                                                 Polyethylene glycol 400 (anti-wicking agent)                                                           4.44                                                 Sodium carbonate (buffer)                                                                              0.44                                                 ______________________________________                                    

All these examples were used with a nozzle diameter of 50 microns and amodulation of 10⁵ Hz. The table below illustrates the properties ofthese inks and illustrates their suitability for use in ink jet printers

    ______________________________________                                                  Example No.                                                                   1          2       3                                                ______________________________________                                        pH          8.5          7.5     7.7                                          Conductivity                                                                              3200         361     410                                          (μmhos/cm)                                                                 Plastic viscosity                                                                         2.8          1.7     8.2                                          (mPas)                                                                        Yield value 0            0.4     0                                            (Pa)                                                                          ______________________________________                                    

The invention thus provides a wide range of pigmented inks for use inink jet printers for decorating ceramics and glass. Inks of theinvention are particularly suited for use in printing on ceramicproducts where the ink may comprise in addition to a pigment a fluxwhich must also meet the aforementioned particle size distributionrequirements.

According to a preferred embodiment, the pigment of the ink isferromagnetic to permit anything written therewith to be read by amagnetic sensor.

The inks outlined above are given for example purposes only and manyother modifications may be made without departing from the scope of theinvention.

Whilst endeavouring in the foregoing specification to draw attention tothose features of the invention believed to be of particular importanceit should be understood that the Applicant claims protection in respectof any patentable feature or combination of features hereinbeforereferred to and/or shown in the drawings whether or not particularemphasis has been placed thereon.

We claim:
 1. An ink comprising an inorganic pigment, wherein the ink isusable in an ink jet printer for printing onto ceramics or glass toprovide a decoration thereon following subsequent firing, said inkfurther comprising a flux for preventing degradation of the ink duringfiring, the maximum particle size of the pigment and the flux beingsufficiently small not to block the nozzles or the filters of theprinter, and the particle size range being sufficiently narrow for theink to have a low enough viscosity for the printer to operate.
 2. An inkaccording to claim 1, wherein the median particle size of the pigment isbetween 0.2 and 2 microns.
 3. An ink according to 1 or 2, wherein thepigment has a surface area of between 4 and 30 m² /g.
 4. An inkaccording to claim 1, wherein for use with a 50 micron nozzle themaximum particle size of the pigment is below 4 microns.
 5. An inkaccording to claim 1, wherein for use with a 50 micron nozzle less than20% of the particles of pigment are less than 0.2 microns.
 6. An inkaccording to claim 1, wherein the pigments in the ink are other thancarbon black or titania based.
 7. An ink according to claim 1, whereinthe proportion of pigment in the ink is in the range 1-85% by weight. 8.An ink according to claim 7, wherein the proportion of pigment in theink is in the range 20-60% by weight.
 9. An ink according to claim 1,wherein the ink comprises greater than 15% by weight of a medium.
 10. Anink according to claim 9, wherein the medium is a polar solvent.
 11. Anink according to claim 10, wherein the medium is any of water, methylethyl ketone, alcohol or a mixture of these.
 12. An ink according toclaim 1, wherein the ink comprises one or more of the followingconstituents in the range given:

    ______________________________________                                                           WT. %                                                      ______________________________________                                        Surfactant/Dispersing agent                                                                         0-20                                                    Suspending Agent      0-20                                                    Film Former/Binder/Hardner                                                                          0-20                                                    Humectant             0-20                                                    Chelating agent      0-1                                                      Biocide              0-1                                                      Defoaming agent      0-1                                                      Buffering agent      0-1                                                      Anti wicking agent    0-20                                                    or                                                                            Substrate penetrant   0-5.                                                    ______________________________________                                    


13. An ink according to claim 12, wherein the surfactants/dispersingagents are wetting agents.
 14. An ink according to claim 12, wherein thesurfactants/dispersing agents comprise, for aqueous systems, sodiumpolyphosphate, sodium carboxylic acid, and mixtures thereof.
 15. An inkaccording to claim 12, wherein the surfactants/dispersing agentscomprise, for organic systems, triethanolamine, a dimethylethanolaminesalt, and mixtures thereof.
 16. An ink according to claim 12, whereinthe film formers/binders/hardener comprise one or more of resins,rosins, esterfied resins, acrylic, styrene acrylic, cellulose, amides,epoxies.
 17. An ink according to claim 12, wherein the humectant ispolyethylene glycol (PEG).
 18. An ink according to claim 12, wherein thechelating agent comprises EDTA.
 19. An ink according to claim 12,wherein the biocides comprise formalin.
 20. An ink according to claim12, wherein the biocides comprise one of the acticide range of biocides.21. An ink according to claim 12, wherein the defoaming agents compriselow molecular weight silicones.
 22. An ink according to claim 12,wherein the pigment is ferromagnetic to permit anything writtentherewith to be read by a magnetic sensor.